Fibroblast Activation Protein Activates Macrophages and Promotes Parenchymal Liver Inflammation and Fibrosis

Fibroblast activation protein is highly expressed on a subset of activated fibroblasts and stellate cells at mesenchymalepithelial interfaces and is associated with macrophage infiltration. Specific fibroblast activation protein inhibition modulates macrophage function, attenuates parenchymal liver fibrosis, and facilitates hepatocyte proliferation, with reduced efficacy in biliary fibrosis. BACKGROUND & AIMS: Fibroblast activation protein (FAP) is expressed on activated fibroblast. Its role in fibrosis and des-moplasia is controversial, and data on pharmacological FAP inhibition are lacking. We aimed to better define the role of FAP in liver fibrosis in vivo and in vitro. METHODS: FAP expression was analyzed in mice and patients with fibrotic liver diseases of various etiologies. Fibrotic mice received a specific FAP inhibitor (FAPi) at 2 doses orally for 2 weeks during parenchymal fibrosis progression (6 weeks of carbon tetrachloride) and regression (2 weeks off carbon tet-rachloride), and with biliary fibrosis (Mdr2-/-). Recombinant FAP was added to (co-)cultures of hepatic stellate cells (HSC), fibroblasts, and macrophages. Fibrosis-and inflammation -related parameters were determined biochemically, by quan-titative immunohistochemistry, polymerase chain reaction, and transcriptomics. RESULTS: FAP+ fibroblasts/HSCs were a-smooth muscle actin (a-SMA)-negative and located at interfaces of fibrotic septa next to macrophages in murine and human livers. In parenchymal fibrosis, FAPi reduced collagen area, liver collagen content, alpha- SMA+ myofibroblasts, M2-type macrophages, serum alanine transaminase and aspartate aminotransferase, key fibrogenesis-related transcripts, and increased hepatocyte proliferation 10-fold. During regression, FAP was suppressed, and FAPi was ineffective. FAPi less potently inhibited biliary fibrosis. In vitro, FAP small interfering RNA reduced HSC a-SMA expression and collagen production, and FAPi suppressed their activation and proliferation. Compared with untreated macro-phages, FAPi regulated macrophage probrogenic activation and transcriptome, and their conditioned medium attenuated HSC activation, which was increased with addition of recom-binant FAP. CONCLUSIONS: Pharmacological FAP inhibition attenuates inammation-predominant liver brosis. FAP is expressed on subsets of activated broblasts/HSC and promotes both macrophage and HSC probrogenic activity in liver brosis. (Cell Mol Gastroenterol Hepatol 2023;15:841-867; https:// doi.org/10.1016/j.jcmgh.2022.12.005)